Barrelene derivatives

Aromatic compounds can participate in both [2+2] and [4+2] photocycloaddition reactions with uracil derivatives to give either benzocyclobutane or ethenoquinazoline (barrelene) derivatives, which can then undergo a number of subsequent photochemical reactions. The products obtained are dependent upon the reaction conditions, and thus the photocycloaddition reaction between naphthalenes 470 and l,3-dimethyl-5-fluorouracil 471 in cyclohexane gave 4a-fluoro-5,10-ethenobenzo[/]quinazolines 472 as products as a result of a [4+2] photocycloaddition (photo-Diels-Alder) reaction <2002TL3113, 2003H(61)377>. 

All pnictohetarenes with Y = P, As, Sb, and Bi react with dienophiles such as hexafluoro-2-butyne affording hetero-barrelene derivatives (P- at 100 °C, As- at 25 °C, Sb- and Bi-pnictohetarenes at even lower temperatures). The equilibrium low-temperature dimerizations of bismabenzene and stibabenzene (37-38) also involve such cycloadditions. 

Steady state and laser-flash photolysis have been used to study the photochemical transformation of the barrelene derivative (47). Irradiation of this in a... 

Vinyl-vinyl bridging reactions are also operative in the barrelene derivatives 324. The reaction arises from the triplet and is brought about by xanthene sensitizaion. The semibullvalenes 325 and 326 are formed in a ratio of 8.3 1 in this rearrangement. Even when hetero-atoms are present, there are some instances when vinyl-vinyl bridging is operative. This is demonstrated by the conversion of 327, R = H, into 328. This reaction mode is in competition with the pyrazino-vinyl bridging path that yields 329, R = H, and 330, R = Others have also examined the reactivity of such pyrazine derivatives. The pyrazino-vinyl system is an example of the aza-di-jr-methane system originally discovered by Armesto and his coworkers and reviewed recently by Zimmerman and Armesto . 

The reactions of the strained benzene derivative 25 with DCA were originally studied with the aim of synthesizing planar cyclooctatetraene derivatives which are interesting compounds with respect to the question of their antiaromaticity [56]. The cyclooctatetraene derivative, 29 (Scheme 2.9) synthesized by photolysis of the barrelene derivative 26, however, turned out to be non-planar [57]. But the synthesis and the reactions of barrelene derivative 26 are interesting with respect to the utility of high pressure to control the course of reaction. The reaction of 25 with DCA at 1 bar and 127 °C produces the (1 1) Diels-Alder adduct 26 (yield, 49 %) and the unexpected dark blue (2 1) adduct 27 (yield, 14 %). At high pressure (9 kbar, 83 °C) 27 is the major product even after the low conversion of 36 % of the... 

A study of the di-Ti-methane reactivity of the barrelene derivatives (73) in zeolites has been published. The reaction in a slurry affords a 77 23 mixture of (74) and (75) when the reaction is carried out in a zeolite the cyclooctatetraene product is suppressed and the two products are obtained in a ratio of 1 99. This enhancement of the di-Ti-methane reactivity occurs with Li" - and Na -exchanged zeolites.Liao and co-workers have reported new reactivity of some barrelenes. The reactions encountered are sensitive to substitution pattern. Thus, the irradiation of (76) with electron withdrawing groups follows the di-TT-methane route, to yield (77) and (78) predominantly. The less heavily substituted derivative (79) behaves differently, and irradiation affords (80) and (81) by the aza-di-7r-methane rearrangement, with (82) formed only in small amounts by the alternative di-Ti-methane path. Calculations have been used to examine the mechanism of the barrelene-semibullvalene isomerization. These results indicate that two biradical intermediates are involved in the T state. Other calculations on the di-Ti-methane rearrangement of barrelene substantiate the Zimmerman mechanism for the sensitized rearrangement. 

The same concept has been extended to the synthesis of aromatic and heteroaromatic compounds bearing two 1-adamantyl substituents <90JA5654> and two neopentyl substituents <94TL2709> at ort/io-positions. Interestingly, the reaction of 3,4-dineopentylthiophene-1,1-dioxide with benzyne (from 2-carboxybenzenediazonium chloride) gives (217) as the major product (68%) via an ene reaction the cycloadduct-S02 extrusion product (218) is formed in 20% yield. The latter undergoes further cycloaddition with another molecule of benzyne to give a dibenzo barrelene derivative (7.5%). The formation of such barrelene derivatives had been observed earlier with a few other thiophene-1,1-dioxides as well <86H(24)1233>. 

Other Octacarbonyidicobalt-Catalyzed Reactions. A retro-Diels-Alder reaction catalyzed by Co2(CO)8 was observed in which a barrelene derivative loses a C2H2 fragment (eq 30). An analogous cleavage accompanies the cyclopentenone synthesis (see below) when norbomadiene reacts in certain solvents with alkynehexacarbonyldicobalts, as shown by the formation of dicarbonylcyclopentadienylcobalt, whereas a Diels-Alder addition catalyzed by a cobalt carbonyl species is involved when the same reaction is applied to cyclohexadiene. ... 

Strong acids or superacid systems generate stable fluorinated carbocations [40, 42] Treatment of tetrafluorobenzbarrelene with arenesulfonyl chlorides in nitro-methane-lithium perchlorate yields a crystalline salt with a rearranged benzo barrelene skeleton [43] Ionization of polycyclic adducts of difluorocarbene and derivatives of bornadiene with antimony pentafluonde in fluorosulfonyl chloride yields stable cations [44, 45]... 

Occidentalol (628), a eudesmane-type sesquiterpene isolated from the wood of Eastern white cedar, was synthesized from a as-fused decalin system (627) prepared by cycloaddition of the a-pyrone (625) with 4-methyl-3-cyclohexenone (626 Scheme 140) (72TL4651). Derivatives of 2-pyrone have also been employed in syntheses of colchicine (629 Scheme 141) (59AG637) and barrelene (630 Scheme 142) (69JA2330). 

Di-ir-Methane Rearrangement of Barrelene, Benzobarrelene, Dibenzobarrelene, and Related Derivatives... 

Barrelene, benzobarrelene and dibenzobarrelene derivatives are readily synthesized by Diels-Alder reactions of arenes with alkenes. For barrelene and related derivatives, the DPM rearrangement proceeds solely from the triplet state, giving semibullva-lenes (Scheme 4.1) [3, 4]. 

Aside from simple processes such as cis-trans isomerization, the first example of a potential energy surface derived for a photochemical rearrangement or reaction was in 1967. This was work by the Zimmerman group 44-47) in which the hypersurface for the Di-u-Methane rearrangement of barrelene to semibullvalene was obtained note Fig. 13. 

Bridges other than methylene, e.g. those present in benzobarrelene and its dihydro derivative, also provide for reactive di-ir-methane systems. " In the benzobarrelene case, aryl-vinyl or vinyl-vinyl bonding is possible. As shown in equation (15), the products derived from these two pathways are distinctive for the labelled substrate. This aspect has been studied for several barrelenes containing different aromatic groups, including naphthalene (both at the 1,2- and 2,3-positions), anthracene, etc. The observed products are rationalized most frequently via vinyl-vinyl bridging. Similar behavior is exhibited by diben-zobarrelene and related systems. ... 

Asymmetric induction in the di-jr-methane rearrangement is also of interest and studies on this have examined the influence of chiral esters. Thus the irradiation of 370 yields a cyclooctatetraene 371 and a diastereoisomeric mixture of the semibullvalenes 372 and 373 in a ratio of 60 40 in solution and 20 80 in the solid phase. The position of attachment is vital and the diastereoselectivity shown by the barrelene 374 is very poor and yields a 1 1 mixture of products . Further examples of the control exercised when dibenzobarrelenes are inadiated in the crystalline phase have used the derivatives 375 and 376 as the acid salts formed with chiral amines. Irradiation affords the products 377 and 378 respectively, obtained after esterification with MeOH, with an ee of >95% . The influence of a chiral crystal lattice on the outcome of the di-jr-methane reaction of achiral 379 has been studied. The irradiation in the crystalline phase gives two chiral di-TT-rnethane products 380 and 381. The former of these is racemic but the latter is obtained in high enantiomeric excess which, under the best conditions, i.e. at —20 °C, approaches 100%. The irradiation of ciystals of 345, as an ethanol complex, affords 382 with an ee of 94%. Carrying out the inadiation at temperatures lower than ambient enhances the specificity of the reaction . 

A study of the photochemical reactivity of the pyrazine derivatives of barrelene (95) and (96) has shown that the di-a-methane process is dominated by pyrazino-vinyl bondins made. Irradiation of the derivative (95, R s H) yields the products (96) by a vinyl-vinyl bridsins path and (97, R = H) and (98, R a H) froa the pyrazino-vinyl bridsins- This, (95, R a H), is the only compound in the group which shows this reactivity. Compound (95, R a CN) yields only products (97, R a CN) and (98, R = CN) from the pyrazino-vinyl path. When the more sterically crowded system (99) was used irradiation save the products (100a)and (100b).The former is produced in 76X yield and is the result of... 

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